Engines may combust a mixture of air and fuel to generate torque. A ratio of air to fuel, referred to as the air-fuel ratio or fuel-air ratio, may be controlled responsive to feedback from various sensors, including exhaust gas oxygen sensors. Closed loop control of the engine air-fuel ratio may be composed of several control loops: an inner loop that seeks to regulate the exhaust gas before it passes through an emission reducing catalyst, and an outer loop that uses measurements of the gas after it passed through the catalyst.
The inner loop control may have several control objectives, including maintaining the feed-gas (engine out) air-fuel ratio to reduce emissions, reduce fuel economy losses, and reduce NVH or drivability issues. Additionally, the inner loop may aim to regulate the feed-gas fuel-air ratio to track a target value set by operating conditions such as engine speed, load, temperature, etc., and modified by the outer loop feedback. The outer loop may operate to adjust the inner loop fuel-air ratio target based on post catalyst sensor readings that indicates the catalyst state. The outer loop feedback control faces various challenges predominantly due to a long delay before any feed-gas change at the input of the catalyst is seen at the output and measured by the HEGO sensor.
It has been proposed to augment the inner loop to address large propagation delays and dynamic lags that exist in the combustion/exhaust system, such as described in U.S. Pat. No. 7,987,840). Additionally, an additional integral term can be added to a standard proportional-integral (PI) controller used in the inner loop to track disturbances that were not rejected upstream of the catalyst. The tracking integrator can be placed into the controller structure (for example in series with the original integrator); however this will lead to conflicts if an anticipatory controller (e.g., a delay compensator such as a Smith Predictor), is used.
The inventors have recognized the above-described disadvantages and, in embodiments provide an engine method, comprising adjusting fuel injection from an anticipatory controller responsive to exhaust oxygen feedback of an exhaust gas sensor positioned upstream of an exhaust catalyst, the anticipatory controller including a first integral term and a second integral term, the second integral term correcting for past fuel disturbances.
In this way, it is possible to more accurately maintain the fuel-air ratio entering the exhaust catalyst at stoichiometry on average over time, by cancelling previous errors with later corrections. Normally such corrections are countered by the anticipatory controller. However, by placing an additional integrator in the inner loop in a reference location of the anticipatory controller, the time-integrated average air-fuel ratio in the exhaust catalyst can be controlled even in the presence of one-sided (e.g., asymmetric) disturbances. Additionally, the additional integrator may be clipped based on engine torque disturbance limits and based on whether the exhaust catalyst is, or is about to be, saturated with stored oxygen, or depleted of stored oxygen.
In one particular example, the method may structure the inner loop controller to track a ramp type input, which may be effective in dealing with the above-mentioned fuel disturbance problems. The additional integrator term integrates the error and adds this to the controller output so as to counteract disturbances that have already occurred, as long as the catalyst is operated in a non-saturated state. As such, the challenges to the outer loop control are reduced by action the inner loop controller takes to keep the catalyst oxygen storage within a desired range. Specifically, it is possible to deal with fueling disturbances that occur by altering the reference set point to make up for the disturbance over a period of time. By countering this known disturbance soon after it occurs while still enabling predictive controller action, the impact on the catalyst is reduced, making outer loop control less difficult.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure. Further, the inventors herein have recognized the disadvantages noted herein, and do not admit them as known.